Learning how axons of retinal ganglion cells grow from their origins in the retina to their targets in the brain is an important goal for understanding how the visual system develops. A number of cell adhesion molecules (CAMs) have been found in the optic fiber layer and optic nerve and are thought to be important in axon growth and guidance. A basic idea in the field has been that CAMs mediate cell-cell interactions via cell-cell binding. Over the past few years a more complex concept has evolved, i.e., that CAMs not only mediate cell adhesion but are also capable of signaling by influencing intracellular second messenger systems. One CAM, called L1, is present on axons of retinal ganglion cells. We have shown that L1 is a potent substrate for growth of retinal ganglion cell axons in vitro. We have also identified several kinases that interact with L1 and are activated by L1 signaling. The L1 cytoplasmic domain (L1CD) is 114 amino acids long and is identical in mice and humans. Mutations of the L1 gene in humans and mice cause X-Iinked hydrocephalus and disrupt development of the visual system, including the retina. Interestingly, mutations that only affect the L1CD also cause brain abnormalities.The experiments in this proposal will use biochemical, immunological, molecular biological and cell biological experiments to study the function of the L1CD. We will determine if L1 heterophilic binding partners activate different signaling pathways than L1 homophilic binding. We will examine how phosphorylation and palmitoylation of L1 is regulated and how these post-translational modifications influence L1 function, especially interactions with other intracellular proteins. This aim includes yeast 2-hybrid approaches and proteomics. Finally, site directed mutagenesis will be performed on the cytoplasmic domain of L1 to clarify how they participate in neurite growth in vitro. These experiments will provide new information about how cell adhesion modulates intracellular processes and also how changes in intracellular second messenger systems can alter cell adhesion. Finally, these experiments will provide a detailed description of how one cell adhesion molecule regulates growth cone behavior.